<template>
  <div>
  <div :class="arr">
    你好世界
   <button @click="arr.push('mid')">点我加类名</button>
  </div>
   
  </div>
</template>

<script>
export default {
  name: 'VueDemoApp',

  data() {
    return {
      arr:['red','sm']
    };
  },


  methods: {
    
  },
};
</script>

<style >
div{
  width: 200px;
  height: 200px;
  background-color: rgb(255, 104, 104);
}
</style>